Wye and Delta Connections of Single-Phase Transformers

• diagram the connection of three single-phase transformers to form a delta-wye
transformer bank.

• describe how a delta-wye transformer bank is used to step down voltages.

• describe how a delta-wye transformer bank is used to step up voltages.

• diagram the connection of three single-phase transformers to form a wye-delta
transformer bank.

• describe how a wye-delta transformer bank is used to step down voltages.

• list advantages and disadvantages of a single three-phase transformer
as compared to three single-phase transformers.

Five commonly-used methods of connecting single-phase transformers to form
three-phase transformer banks are: delta-delta, open delta, wye-wye, delta-wye, and wye-delta connections. The delta-delta, open delta, and wye-wye connections
are described in previous units.

This unit covers the delta-wye and wye-delta connections. The current and voltage relationships for each of these methods of three-phase transformation
are explained, and examples of several applications for each connection method
are shown.

STEP-DOWN APPLICATION FOR DELTA-WYE TRANSFORMER BANK

Assume that electrical energy must be transformed from a 2,400-volt, three-phase,
three-wire input to a 120/208-volt, three-phase, four-wire output. Each of
the three single-phase transformers is rated at 20 kVA, with 2,400 volts
on the high-voltage windings and 120 or 240 volts on the low-voltage windings.

[176-177]

…windings are connected in delta to the generating station bus bars; therefore,
each primary coil winding has 13,800 volts applied to it. The transformers
have a step up ratio of 1 to 5. As a result, the voltage output of the secondary
of each single-phase transformer is 5 x 13,800

or

69,000 volts.

ill 2 shows that the three secondary windings are connected in wye.
Each high-voltage secondary winding is connected between the secondary neutral and one of the three line leads. The voltage between the neutral and any
one of the three line leads is the same as the secondary coil voltage or
69,000 volts. The voltage across the three line leads is 1.73 x 69,000 =
119,370 volts. The grounded neutral wire on the high-voltage secondary output
must be used to obtain balanced three-phase voltages even though the load
current may be unbalanced. Not only is this neutral wire grounded at the
transformer bank, it's also grounded at periodic intervals on the transmission
line. As a result, it protects the three high-voltage secondary windings
of the single-phase transformers from possible damage due to lightning surges.

ill. 2 Delta-wye transformer bank

WYE-DELTA TRANSFORMER BANK

A transformer bank connected in wye-delta is the type most often used to
step down relatively high transmission line voltages (60,900 volts or more)
at the consumer’s location. Two reasons for selecting this type of transformer
bank are that the three-phase volt age is decreased by the transformer ratio
multiplied by the factor 1.73, and the insulation requirements for the high-voltage
primary windings are reduced.

As an example, assume that it's necessary to step down a three-phase 60,900
volt input to a three-phase, 4,400-volt output ( 3). The primary
windings are connected in wye to a three-phase, four-wire transmission line.
The three line voltages are 60,900 volts between phase conductors each and the voltage from each line wire to the grounded neutral is 35,200 volts.

Each of the three single-phase transformers is rated at 1,000 kVA, with
35,200 volts on the high-voltage side and 4,400 volts on the low-voltage
side. The voltage ratio of each transformer is 8 to 1.

ill 3 shows that the secondary windings are connected in delta, resulting
in a line voltage of 4,400 volts on the three-phase, three-wire secondary
system feeding to the load.

ill. 3 Wye-delta transformer bank

The total kVA capacity of a wye-delta transformer bank is determined by
adding the kVA rating of each single-phase transformer in the bank. For the
bank in figure 3, the total kVA capacity is equal to 1,000 + 1,000 + 1,000=-
3,000 kVA.

THREE-PHASE TRANSFORMERS

Voltages on three-phase systems may be transformed using three-phase transformers.
The core of a three-phase transformer is made with three legs. A primary and a secondary winding of one phase are placed on each of the three legs.
These transformers may be connected in delta-delta, wye-wye, delta-wye or
wye-delta. The connections are made inside the transformer case. For delta-delta
connections, three high-voltage and three low-voltage leads are brought out.
Four leads are brought out when any wye-connected windings are used. This
fourth lead is necessary for the neutral wire connection.

The three-phase transformer occupies less space than three single-phase
transformers because the windings can be placed on one core in the three-phase
transformer case, ( 5). The efficiency of a three-phase transformer
is higher than the overall efficiency of three single-phase transformers
connected in a three-phase bank.

ill. 4 A three-phase transformer (Assembled core and coils for a
500-kVA, 60-Hz, 13,800 to 2,400-V transformer) ; ill. 5 34 KV power transformer at a generating station

However, there is one disadvantage to the use of a three-phase transformer.
If one of the phase windings becomes defective, the entire three-phase unit
must be taken out of service. If a single-phase transformer in a three-phase
bank becomes defective it can be replaced quickly. The resultant power interruption
is brief. For this reason, many transformer installations consist of banks
of three single-phase transformers. ill 5 shows a three-phase single
unit transformer in operation.

SUMMARY

Transformers need not be connected in the same pattern on the primary and the secondary. Depending on the desired voltage level and level of step-up
(increase) or step-down (decrease), the patterns may change. The two most
popular patterns are the wye and delta. To get the greatest step-up, the
transformation ratio is best if the primary is connected delta and the secondary
is connected wye. Likewise, to get the largest decrease in voltage, the ratio
of transformation is the greatest if the primary is connected wye and the
secondary is connected delta. Remember that the current ratios are the inverse
of the voltage ratios.

QUIZ

1. Diagram the connections for three single-phase transformers connected
in delta-wye to step down 2,400 volts, three phase, three wire, to a 120/208-volt,
three-phase, four-wire service. Three single-phase transformers are to be
used. Each transformer is rated at 25 kVA, with 2,400 volts on the high-voltage
side and 120 volts on the low-voltage side. Mark leads H X and so forth.
Show all voltages.

2. What is the total kVA capacity of the delta-wye transformer bank in question
1?

3. What are two applications of a three-phase, delta-wye transformer bank?

a.

b.

4. What is one practical application of a three-phase, wye-delta transformer
bank?